Todd H. Hubing
Clemson Vehicular Electronics Laboratory
Clemson University
AGND
“Whenever I see more than one of
these symbols on the schematic, I
know there is [EMC] work for us
here.”
T. Van Doren
DGND
April 1, 2011
Emerging Technologies in EMC Workshop
Buenos Aires, Argentina
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Chassis GND
 Rarely appropriate
 Often the source of
significant problems
Analog
GND
Digital GND
April 1, 2011
Emerging Technologies in EMC Workshop
Buenos Aires, Argentina
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Digital GND
Analog GND
Chassis GND
 Only one plane usually needs to be full size.
 One or zero vias should connect planes with different labels.
April 1, 2011
Emerging Technologies in EMC Workshop
Buenos Aires, Argentina
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 Why do I need more than one ground?
 Where does each ground need to be?
 How do I connect the grounds?
April 1, 2011
Emerging Technologies in EMC Workshop
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1. Current always returns to its source!
2. Current takes the path of least impedance!
> 100 kHz this is generally the path of least inductance
< 10 kHz this is generally the path(s) of least resistance
April 1, 2011
Emerging Technologies in EMC Workshop
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Where does the 56 MHz return current flow?
April 1, 2011
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Where does the 1 kHz return current flow?
Analog
Amplifier
POWER
SUPPLY
4 MHZ
OSC.
Digital / Analog
Photo
Transistor
1 kHz Analog Signal
Fiber
optic
input
power input
and
high-speed
signal output
Connection to power plane
Connection to ground plane
April 1, 2011
Emerging Technologies in EMC Workshop
Buenos Aires, Argentina
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April 1, 2011
t
f
t
f
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The purpose of a system ground is to provide a reference
voltage and/or a safe path for fault currents.
Signal currents flowing on a “ground” conductor can prevent a
ground conductor from serving its intended purpose.
Don’t confuse ground conductors with signal return
conductors. Rules for the routing of “ground” may conflict
with the rules for routing signal or power returns.
April 1, 2011
Emerging Technologies in EMC Workshop
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Single-Point Ground
Multi-Point Ground
These are grounding strategies, not signal return strategies!
April 1, 2011
Emerging Technologies in EMC Workshop
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Ground plane
April 1, 2011
Emerging Technologies in EMC Workshop
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Signal return plane
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Emerging Technologies in EMC Workshop
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Plane serves dual roles
April 1, 2011
Emerging Technologies in EMC Workshop
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Current Driven Radiation Mechanism
Signal current loop induces a voltage between two good antenna parts.
- Vcm +
A few millivolts of common-mode potential driving two cables is
sufficient to exceed CISPR Class B radiated emissions limits.
April 1, 2011
Emerging Technologies in EMC Workshop
Buenos Aires, Argentina
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Analog
GND
April 1, 2011
Chassis GND
Digital GND
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Circuit boards should have
high-frequency ground!
Why?
Conductors referenced to different grounds can be good antennas.
Signals referenced to two different grounds will be noisy (i.e. include the
noise voltage between the two grounds).
Layouts with more than one ground are more difficult, require more
space and present more opportunities for critical mistakes.
Excuses for employing more than one ground are generally based on
inaccurate or out-dated information.
April 1, 2011
Emerging Technologies in EMC Workshop
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If grounds are divided, it is generally to control the flow of low-frequency
(<100 kHz) currents.
For example,
Isolating battery negative (i.e. chassis ground) from digital ground
Isolating digital ground from analog ground in audio circuits.
This can be necessary at times to prevent common impedance coupling
between circuits with low-frequency high-current signals and other
sensitive electronic circuits.
HOWEVER, it is still necessary to ensure that there is only 1 high-frequency
ground.
April 1, 2011
Emerging Technologies in EMC Workshop
Buenos Aires, Argentina
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Exercise: Trace the path of the digital and analog return currents.
D/A
April 1, 2011
Emerging Technologies in EMC Workshop
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Exercise: Trace the path of the digital and analog return currents.
D/A
April 1, 2011
Emerging Technologies in EMC Workshop
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Exercise: Trace the path of the digital and analog return currents.
D/A
April 1, 2011
Emerging Technologies in EMC Workshop
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Design Exercise: What is wrong with this design and how would you improve it?
April 1, 2011
Emerging Technologies in EMC Workshop
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You don’t need to gap a plane to control the flow of high frequency
(>1MHz) currents. If you provide a low-inductance path for these currents
to take, they will confine themselves to this path very well.
April 1, 2011
Emerging Technologies in EMC Workshop
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Rules for gapping a ground plane:
1. Don’t do it!
2. If you must do it, never ever allow a trace or another plane to cross
over the gap.
3. If you must do it, never ever place a gap between two connectors.
4. Remember that the conductors on either side of the gap are at
different potentials.
5. See Rule #1!
April 1, 2011
Emerging Technologies in EMC Workshop
Buenos Aires, Argentina
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¡Ay, caramba!
April 1, 2011
Emerging Technologies in EMC Workshop
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April 1, 2011
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If you have analog and digital returns that must be
isolated (to prevent common-impedance coupling):
 Route the returns on separate conductors
 Provide a DC connection at the one point (or in the one
area) where the reference potential must be the same.
 This must include every place where a trace crosses the
boundary between the analog and digital regions.
April 1, 2011
Emerging Technologies in EMC Workshop
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Example: How would you modify this design?
April 1, 2011
Emerging Technologies in EMC Workshop
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Example: A much better design
April 1, 2011
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April 1, 2011
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ONE VIA
Analog GND
Digital GND
Digital GND
If you think you need two vias, then you shouldn’t be isolating
the analog and digital grounds.
April 1, 2011
Emerging Technologies in EMC Workshop
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Cables and enclosures are both good antenna parts. If they are not held
to the same potential, they are likely to create a radiation problem.
Exceptions:
When there is no chassis ground
When there are no connectors with cables
Note: Sometimes low-frequency isolation between chassis and digital ground is
necessary control the flow of low-frequency currents. However, even in these
situations it is usually important to provide a good high-frequency connection.
April 1, 2011
Emerging Technologies in EMC Workshop
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Chassis connection
to chassis ground
Wiring Harness
Chassis connection
to chassis ground
Capacitors connecting
chassis ground to the
digital return plane
Digital Return Plane
Chassis Ground Plane
April 1, 2011
Emerging Technologies in EMC Workshop
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 Identify your HF ground and be sure it is the only ground
that is large or connected to anything large!
 Don’t call anything other current carrying nets “ground”.
For example, refer to a current carrying analog reference
net as “analog return”.
 Be aware of where your HF and LF currents are flowing!
 Isolate returns only when necessary to control the flow
of low frequency currents.
 If you isolate two large conductors at low frequencies,
be sure they are well connected at high frequencies.
April 1, 2011
Emerging Technologies in EMC Workshop
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http://www.cvel.clemson.edu
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